Strobe light and laser beam detection for laser receiver

ABSTRACT

A device and method detect a moving, relatively thin beam of laser light, and distinguishing between illumination of photodetectors by the beam, and illumination of the photodetectors by an omnidirectional pulse of light. A plurality of photodetector arrays are arranged in a generally vertical row, with each array having a generally vertical row of photodetector elements. Weighting circuits associated with the arrays provide first and second reference signals for each array related to the elements illuminated in the array. The ratio of sum of the reference signals to the difference between the largest and smallest of the reference signals is determined and interpreted as indicating illumination by a relatively thin beam of laser light when the ratio is less than a predetermined level, and as indicating illumination by an omnidirectional pulse of light when the ration is greater than the predetermined level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/936,617 filed Sep. 8, 2004.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates to equipment of the type used in surveyingand construction and, more particularly, to a detector device for suchapplications which has an improved photodetector arrangement fordetecting the position of reference light. The light may typically be arotating laser beam that defines a reference plane of light or astationary reference plane or cone of laser light. The reference planemay be horizontal or inclined, as dictated by the application.Additionally, a transmitter projecting the reference light may alsoregularly project a strobe light in synchronization with rotation of thebeam so that the beam defines a specific azimuth angle.

Laser systems have commonly been employed in surveying and constructionin which a laser beam is rotated in either a horizontal or a gradedplane. U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando,illustrates a laser transmitter which provides such a rotating referencebeam. The rotating beam defines a plane, and various measurements can bemade using the plane as a reference. For example, the elevation of apoint remote from the laser transmitter may be measured using a rod onwhich a laser receiver is mounted. The bottom of the rod rests on theground at the point where the measurement is to be made, and theoperator moves the receiver along the rod to a position where itintercepts the laser beam, as indicated by a display on the receiver.One such laser receiver is shown in U.S. Pat. No. 4,240,208, issued Jun.30, 1987, to Pehrson.

Additionally, laser systems have provided an indication of the azimuthangle of the laser beam relative to a reference direction (for example,true north) by transmitting a pulse of light, for example a strobepulse, once during each rotation when the beam is directed in thereference direction. Such a system is shown, for example, in U.S. Pat.No. 6,643,004, issued Nov. 4, 2003, to Detweiler et al. Further, it iscommon for strobe lights to be used as warning lights at a constructionsite while machine control and surveying operations are taking place.

A laser receiver operating in such an environment must be able todistinguish between those times when the receiver is illuminated by thelaser beam and those times when it is illuminated by a pulse of lightfrom a strobe source. One way to distinguish between the two is toprovide separate detector elements for the laser light and the strobelight, with optical filters to screen out all light except for light atthe appropriate frequencies. This may not always be completelyeffective, however, and in addition to the unwanted interference, theoptical filters may also attenuate light at the frequencies of interest,making such light harder to detect.

It is seen, therefore, that there is a need for a device and method forutilizing a plurality of photodetector arrays including a large numberof photodetectors, and for accurately defining which one or ones of thephotodetectors have been illuminated and by which light source thephotodetectors have been illuminated by processing a number of outputsignals that have a continuously varying relationship.

SUMMARY OF THE INVENTION

This need is met by a method according to the present invention fordetecting a moving, relatively thin beam of laser light, anddistinguishing between illumination of photodetectors by such a moving,relatively thin beam of laser light, and illumination of photodetectorsby an omnidirectional pulse of light. This method includes the steps ofpositioning a plurality of photodetector arrays in a generally verticalrow, with each array including a plurality of photodetector elementsarranged in a generally vertical array row. The photodetector elementsin the arrays are collectively arranged in a generally verticallyoriented, extended row. Each of said photodetector elements provides anelectrical output when illuminated. For each array, a portion of theelectrical output of each photodetector element in the array is providedas a first reference signal related to the spacing of the photodetectorelement from a first end of the array row, and as a second referencesignal related to the spacing of the photodetector element from thesecond end of the array row. The sum of the first and second referencesignals from said photodetector arrays is determined, as is thedifference between the largest and smallest of the first and secondreference signals from said photodetector arrays. The sum of the firstand second reference signals from said photodetector arrays is dividedby the difference between the largest and smallest of the first andsecond reference signals from said photodetector arrays and the smallestof the first and second reference signals from said photodetectorarrays. The resulting quotient is then interpreted as indicatingillumination of the photodetectors by a moving, relatively thin beam oflaser light when the quotient is less than a predetermined level, andinterpreting the quotient as indicating illumination of saidphotodetectors by an omnidirectional pulse of light when said quotientis greater than this predetermined level. In the event that the quotientindicates illumination of said photodetectors by the moving, relativelythin beam of laser light, the position of the beam of laser light isdetermined with respect to the extended row based on the levels of thefirst and second reference signals from each of the arrays.

A device for detecting a moving, relatively thin beam of laser light,and distinguishing between illumination of photodetectors by such amoving, relatively thin beam of laser light, and illumination ofphotodetectors by an omnidirectional pulse of light, includes aplurality of photodetector arrays arranged in a generally vertical row.Each array includes a plurality of photodetector elements arranged in agenerally vertical array row, such that the photodetector elements inthe arrays are arranged in a generally vertically oriented, extendedrow. Each of the photodetector elements provides an electrical outputwhen illuminated. A plurality of weighting circuits are provided, eachweighting circuit associated with a respective one of the plurality ofphotodetector arrays. Each weighting circuit provides a portion of theelectrical output of each photodetector element in the associated arrayas a first reference signal related to the spacing of the photodetectorelement from a first end of the array row, and provides a portion of theelectrical output of each photodetector element in the associated arrayas a second reference signal related to the spacing of the photodetectorelement from the second end of the array row. The weighting circuitassociated with each array row is connected to the weighting circuitsassociated with adjacent array rows. Circuitry is associated with andresponsive to the reference signals from connected weighting circuitsfor determining the ratio of sum of the reference signals to thedifference between the largest and smallest of the reference signals,and interpreting this ratio as indicating illumination of thephotodetectors by a moving, relatively thin beam of laser light when theratio is less than a predetermined level, and interpreting the ratio asindicating illumination of the photodetectors by an omnidirectionalpulse of light when the quotient is greater than the predeterminedlevel.

Accordingly, it is an object of the present invention to provide adevice and method for detecting the relative position of referencelight; to provide such a device and method in which a plurality ofphotodetector arrays are positioned such that the photodetector elementsin all of the arrays are arranged in an extended row; and to providesuch a device and method in which a plurality of weighting circuits areresponsive to the arrays and a plurality of output circuits areresponsive to the weighting circuits for determining the position of thereference light, and for determining whether the device is illuminatedby a laser beam or by a strobe light pulse.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of the circuitry which formsthe device of the present invention;

FIG. 2 is a schematic diagram of a portion of the photodetector arraysand weighting circuits of the device of FIG. 1, showing four arrays ofphotodetectors connected to four concatenated weighting circuits;

FIG. 3 is a schematic diagram of a portion of the circuitry which formsthe device of the present invention, showing m arrays connected to mconcatenated weighting circuits; and

FIG. 4 is a flow chart illustrating the process by which the system ofthe present invention distinguishes between illumination by a pulse ofstrobe light and illumination by a relatively narrow beam of laserlight.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is made to FIG. 1, which generally illustrates a portion of adetection device 10 according to the present invention. The devicedetects reference light, such as laser light, and provides an indicationof the location of the reference light with respect to the device. Thedevice may also detect reference strobe pulses, as well. Reference lightmay, for example, be produced by laser transmitters as shown in U.S.Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando et al, or in U.S.Pat. No. 4,732,471, issued Mar. 22, 1988, to Cain et al. Other lighttransmitters may also be used.

The detector device 10 includes a display 12, and a photodetectorarrangement 14 including a plurality of photodetector arrays 15 ¹, 15 ²,15 ³, and 15 ⁴, shown in FIG. 2, arranged in a generally vertical row.Although for purposes of FIGS. 1 and 2 four photodetector arrays areshown, a greater or lesser number may be positioned in an extended rowto permit light detection over a larger or lesser dimensional range.Each array includes a plurality of photodetector elements, CR1 throughCR12, arranged in a generally vertical array row. The photodetectorelements in these arrays 15 ¹, 15 ², 15 ³, and 15 ⁴, are collectivelyarranged in a generally vertically oriented, extended row. It will beappreciated that the terms “vertical” and “horizontal” are used here inonly a relative sense, and are not intended to refer to any particularabsolute orientation. The detector device may be operated with itsextended row of photodetector elements in any orientation; it ispreferable, however, that the row of photodetector elements be generallyperpendicular to the reference plane of laser light which is to bedetected. Each of the photodetector elements CR1-CR12 comprises a PINdiode that provides an electrical output when illuminated with thereference light. When the photodetector arrangement 14 is illuminated,output circuits 22 provide an indication on display 12 of the positionof the reference plane of light with respect to the detector device 10.As an example, the display 12 may indicate whether the light is above,below, or within a reference band. Alternatively, the display 12 mayprovide more precise information regarding the position of the referencelight, such as a numerical indication of position, or a multi-leveldisplay. Other display arrangements may be used or, alternatively, thedevice may be provided with no display, and with the output from theoutput circuits 22 being utilized by a machine control circuit tocontrol machine position automatically.

The output circuits 22 are responsive to the relative levels of thefirst and second reference signals on lines 23 and 24, respectively,provided by the photodetector arrangement 14 for determining theposition of the reference light. The output circuits 22 are alsoresponsive to levels of reference signals on lines 25 and 26 fromphotodetectors in the other arrays.

he device further includes four weighting circuits 30 ¹, 30 ², 30 ³, and30 ⁴, shown in FIG. 2. As discussed more fully below, weighting circuits30 ¹, 30 ², 30 ³, and 30 ⁴ are associated with photodetector arrays 15¹, 15 ², 15 ³, and 15 ⁴, respectively. It will be appreciated that fewerphotodetector arrays may be utilized if desired. The weighting circuit30 ¹ includes resistors R1-R4 and R6-R13, inductors L1 and L2 (shuntingD.C. sunlight current), and tapped transformer T1. Each of the pluralityof photodetector elements CR1-CR12 is connected to the transformer T1,either directly or through one or more of the resistors. The weightingcircuit 30 ¹ provides a portion of the electrical output of eachphotodetector element, when illuminated, as a component of a firstreference signal on line 23, related to the spacing of the illuminatedphotodetector element from the upper end of the row of photodetectors inthe array 15 ¹. The weighting circuit 30 ¹ also provides a portion ofthe electrical output of each photodetector element, when illuminated,as a component of a second reference signal on line 24, related to thespacing of the illuminated photodetector element from the lower end ofthe row of photodetectors in array 15 ¹. The weighting circuit 30 ¹ isconfigured such that the first reference signal on line 23 increases asthe light moves toward the upper end of the row of PIN diodes, and thesecond reference signal on line 24 increases as the light moves towardthe lower end of the row. Conversely, the first reference signal on line23 decreases as the light moves toward the lower end of the row of PINdiodes CR1-CR12 in array 15 ¹, and the second reference signal on line24 decreases as the light moves toward the upper end of the row. Whenthe light is in the middle of the row, the levels of the two referencesignals on lines 23 and 24 are equal. The output circuits 22 respond tothe levels of the first and second reference signals on lines 23 and 24to determine the position of the reference light. As discussed below,the output circuits 22 respond to the reference signals on lines 25 and26 to determine the position of the reference light when photodetectorsin other arrays are illuminated.

Each of the weighting circuits 30 ¹, 30 ², 30 ³, and 30 ⁴ is associatedwith a respective one of the plurality of photodetector arrays 15 ¹, 15², 15 ³, and 15 ⁴ and provides a portion of the electrical output ofeach illuminated photodetector element in its associated array as afirst reference signal related to the spacing of the illuminatedphotodetector element from a first end of the array row, and provides aportion of the electrical output of each illuminated photodetectorelement in its associated array as a second reference signal related tothe spacing of the illuminated photodetector element from the second endof the array row. The weighting circuit associated with each array rowis connected to the weighting circuits associated with adjacent arrayrows.

The weighting circuit 30 ¹ provides reference current level signals onlines 23 and 24 in the following manner. Assume that only PIN diode CR3is illuminated. The output current from diode CR3 will be supplied tonode 32 of transformer T1. Approximately ⅚ of the current will passthrough coil 34 to line 23, while ⅙ of the current will pass throughcoils 36, 38, 40, 42, and 44 to line 24. In similar fashion, assume thatthe reference light has moved such that only PIN diode CR9 isilluminated. The output current from PIN diode CR9 will be supplied tonode 46 of transformer T1. Approximately 4/6 of the current from diodeCR9 will pass through coils 42 and 44 to line 24, while 2/6 of thecurrent will pass through coils 34, 36, 38, and 40 to line 23. Thus theway in which the diode current is split and supplied to lines 23 and 24gives an indication of which of the diodes are illuminated and,therefore, the position of the reference light.

It will be noted that a number of the photodetector elements are notconnected directly to a tap of the transformer T1. For example, diodeCR2 has its output supplied to node 48. The current at node 48 is thensplit, with half passing through resistor R1 to line 23, and the otherhalf passing through resistor R2 to node 32. At node 32, ⅚ of this halfof the current passes through coil 34 to line 23, and the remaining ⅙ ofthis half of the passes through coils 36, 38, 40, 42, and 44 to line 24.This results in approximately 11/12 of the current being supplied toline 23.

It will be appreciated that the reference light may commonly illuminatemore than one of the PIN diodes CR1-CR12 of array 15 ¹ at a time. Thismay occur because the size of the reference beam of laser light issignificantly greater than the size of the photodetector elements.Further, thermal gradients in the air through which the laser lightpasses may result in rapid positional fluctuation of the beam,effectively illuminating a number of photodetector elements at once asthe beam flutters between the elements. The weighting circuit 30 ¹splits the current from each of the multiple photodetector elements independence upon the position of the photodetector element in array row.As a consequence, the resulting signal levels on lines 23 and 24 are thesums of the various illuminated photodetector elements, and these signallevels accurately reflect the average position of the middle of thebeam.

The photodetector elements CR1-CR12 in array 15 ¹ are evenly spacedalong a generally vertically oriented row. It is desired that theelements CR1-CR12 be spaced apart by distances that are small ascompared to the dimension of the beam of reference light that is to bedetected. As a consequence, as the light moves vertically with respectto the row of elements it will move onto one photodetector element as itmoves off of the adjacent photodetector element. If the spaces betweenadjacent elements were too large, the levels of the signals on lines 23and 24 might fluctuate undesirably, even though their relativeamplitudes continued to reflect accurately the position of the light.

The output circuits 22 are associated with, and responsive to, referencesignals on lines 23, 24, 25, and 26 from connected weighting circuitsThe output circuits 22 assess the relative levels of the referencesignals from the weighting circuits 30 ¹, 30 ², 30 ³, and 30 ⁴ such thatthe position of the reference light may be determined with respect tothe detector device. Each output circuit preferably includes a separatefilter circuit, current-to-voltage amplifier circuit, and peak detectionand hold circuit for processing the reference signals on lines 23, 24,25, and 26. Preferably, the reference signals are converted to digitalform. The relative levels of the processed reference signals areassessed by output circuits 22, including microprocessor 50, todetermine the position of the reference plane of light with respect tothe row of PIN diodes in arrays 15 ¹, 15 ², 15 ³, and 15 ⁴.

This can be accomplished in a variety of ways. For instance, if therelative levels of the first and second reference signals on lines 23and 24 change linearly as the light moves up and down the row ofphotodetectors in array 15 ¹, the reference light will be spaced alongthe row by a fraction of the row length that is equal to the fraction ofone of the reference signals divided by the sum of the reference signalsone lines 23 and 24. By this technique, it will be appreciated that theabsolute intensity of the light striking the photodetector elements isimmaterial. Rather, it is the relative levels of the two referencesignals from the circuit 30 ¹ that define the position of the referencelight. Alternatively, the ratio of the first and second referencesignals may be taken and used as the address for a table look upalgorithm. It will be appreciated that the position of the referencelight may be categorized in broad or narrow bands, or specified with anylevel of precision desired.

Since the device of FIGS. 1 and 2 includes four photodetector arrays,the position of the light with respect to the extended row defined bythe arrays may be determined as follows. Array 15 ¹ connects toweighting circuit 30 ¹ and lines 23 and 24 on which reference signals A1and A2 are provided, respectively. Array 15 ² connects to weightingcircuit 30 ² and lines 24 and 25 on which reference signals A2 and A3are provided, respectively. Array 15 ³ connects to weighting circuit 30³and lines 25 and 26 on which reference signals A3 and A4 are provided,respectively. Finally, array 15 ⁴ connects to weighting circuit 30 ⁴ andlines 26 and line 27 on which reference signals A4 and A5 are provided.

The position of the reference light can be determined by the formula(A1−A2)/(A1+A2) if it is on the first array 15 ¹, (A2−A3)/(A2+A3) if itis on the second array 15 ², (A3−A4)/(A3+A4) if it is on the third array15, and so on. It is, of course, possible for the reference light toilluminate photodetectors in two adjacent arrays simultaneously, fallingacross the boundary of two adjoining photodetector arrays. This resultsin signals being produced by both arrays. The formulae for theindividual arrays can be combined into one formula which deals withsimultaneous illumination of multiple arrays. For two concatenatedarrays, the combined formula is (2×A1−2×A3)/(A1+A2+A3). For threeconcatenated arrays, the combined formula is(3×A1+A2−A3−3×A4)/(A1+A2+A3+A4). For four concatenated arrays, thecombined formula is (4×A1+2×A2−2×A4−4×A5)/(A1+A2+A3+A4+A5). The formulamay be chosen for any desired number of arrays. The microprocessor 50performs these simple calculations in a straightforward fashion.

FIG. 3 is a schematic diagram, illustrating the generalization of thedevice of the present invention to include m weighting circuits 30 ¹, 30², . . . 30 ^(m), which respond to m corresponding photodetector arrays15 ¹, 15 ², . . . 15 ^(m). As will be appreciated, most photodetectors,such as PIN diodes and phototransistors, are relatively small, and aretherefore spaced on about 0.10 to 0.30 inch centers. Close spacing ofthe photodetectors permits detection of a relatively narrow beam oflaser light, for example. Consequently the number of photodetectorsrequired to build a receiver from about 6 inches up to 6 feet in lengthranges from about 25 to over 250. This would be increased, of course,depending upon the number of rows of receivers that are used to receivelight from different sides of the device. A practical device maytypically include an extended row of photodetectors that is 18 incheshigh and includes seven or eight arrays and seven or eight associatedweighting circuits. Connecting photodetector arrays and weightingcircuits in concatenated fashion produces a relatively small number ofreference signals that nevertheless define the position of the detectedreference light quite precisely. As will be noted, the connections ofthe PIN diodes of FIG. 3 are opposite to those of the PIN diodes of FIG.2, illustrating that either connection arrangement may be utilized inconjunction with appropriate circuitry.

The device described above detects the relative position of a referenceplane of laser light. Initially a plurality of photodetector arrays arearranged in a generally vertical row. Each array includes a plurality ofphotodetector elements also arranged in a generally vertical array row.As a result, the photodetector elements are arranged in a generallyvertically oriented, extended row. Each of the photodetector elementsprovides an electrical output when illuminated with the reference planeof light. A portion of the electrical output of each photodetectorelement in each array is provided as a first reference signal related tothe spacing of the photodetector element from a first end of the arrayrow. A portion of the electrical output of each photodetector element inthe array is provided as a second reference signal related to thespacing of the photodetector element from the second end of the arrayrow. The position of the reference plane of laser light with respect tothe extended row is determined based on the levels of the first andsecond reference signals from each of the arrays.

Portions of the electrical outputs may be separated using a tappedtransformer, with each of the plurality of photodetector elements beingconnected to the transformer. A plurality of tapped transformers areprovided. Each of the plurality of transformers is connected to anassociated photodetector array. The transformers are connected in seriesin the same order as the arrangement of the arrays with which they areassociated in said extended row. The portions of the electrical outputsare separated using a tapped transformer, with each of the plurality ofphotodetector elements being connected to the transformer. For eachphotodetector array, the photodetector elements are connected to thetransformer using one or more resistors.

A number of changes and variations may be made to the device of thepresent invention. For example, the output circuits 22 may be reduced innumber by supplying more than one weighting circuit output to the sameamplifier circuit. The lower of the two outputs 36 from weightingcircuit 30 ^(m) may be supplied in parallel with the output on line 23from weighting circuit 30 ¹. In such an arrangement, the microprocessorwill have no difficulty in distinguishing between the situation in whicharray 15 ¹ is illuminated and the situation in which array 15 ^(m) isilluminated, since in the former a signal will be received on line 23,and in the latter a signal will be received on line 38.

Other variations in the construction and implementation of the device ofthe present invention may be made, as well. The number of arraysconcatenated together can be more or fewer than the implementationshown, to make a longer or shorter receiver. The number ofphotodetectors in each array may be more or fewer than theimplementation shown. This may be accomplished by using differentcomponents to make up the weighting networks, such as transformers witha different number of taps, or using a different arrangement ofresistors between the transformer taps.

If desired, the weighting function may be implement by means other thana transformer. For example, a resistor network may be used. Such anarrangement may require applying a reverse bias voltage to thephotodetectors in the arrays, depending on their parameters and theintensity of the incident light and the ambient light.

As will be appreciated, laser detectors are subject to various errors,including incorrectly identifying an illumination of the detectors by anextraneous light source as a laser strike. This problem is furthercomplicated by transmitters that also produce a periodic pulse of strobelight, typically one pulse of strobe light during each rotation of thelaser beam. Thus, while time windowing (recognizing only beam strikesthat occur during regularly occurring time windows) can be used toexclude from consideration much extraneous, random illumination, itcannot be used to block out light from strobes where the strobes arepulsed at the same frequency as the rotational frequency of the laserbeam.

The present invention is, however, capable of making this distinction.Basically the present invention recognizes that a great many of thephotodetector elements are likely to be illuminated simultaneously by apulse of strobe light, while a much smaller number of the photodetectorelements will be illuminated simultaneously a relatively narrow beam oflaser light.

For example, if there were three detector arrays 15 ¹ through 15 ^(m) inFIG. 3 which were illuminated by a pulse of strobe light, the pulseoutputs on lines 23, 24, 36 and 38 from weighting circuits 30 ¹, 30 ², .. . 30 ^(m) would all be expected to be substantially equal. If, on theother hand, a relatively narrow beam of laser light were to sweep acrossthe detector arrays 15 ¹ through 15 ^(m), it would be expected that atleast one of the pulse outputs on lines 23, 24, 36 and 38 would berelatively low, even if the illumination by the beam occurred over anarea bridging two adjacent arrays.

Consider the case where the maximum weighting circuit outputs over abrief period of time are equal to sA, sB, sC, and sD, and the minimumweighting circuit outputs, generally associated with background lightlevels, are equal to bA, bB, bC, and bD. The processor 50 takes thedifference between maximum and minimum outputs in each case to determinefour reference signals, A, B, C, and D, associated with the arrays 15and weighting circuits 30. The processor then assesses the value ofsignals A, B, C, and D to determine if at least one of these referencesignals is below a predetermined threshold. If all of the signals areabove the threshold then the processor categorizes the illumination asan omnidirectional pulse of light. If, however, at least one of thesignals A, B, C, and D, has a value less than the predeterminedthreshold, then a second test is applied. The processor 50 determinesthe difference between the largest and smallest of the referencesignals, as well as the sum of all of the reference signals. Theprocessor then determines the ratio of the sum of the reference signalsto the difference between the largest and smallest of the referencesignals. This ratio is interpreted as indicating illumination of thephotodetectors by a moving, relatively thin beam of laser light when theratio is less than a predetermined level. This ratio is also interpretedas indicating illumination of the photodetectors by an omnidirectionalpulse of light when said quotient is greater than the predeterminedlevel. It has been found that for four reference signals, apredetermined level which works well is 2.5. When the ratio is less than2.5, a laser beam strike is detected, and when the ratio is above 2.5 astrobe pulse is detected.

Additional filtering may be useful. For example, time windowing may beused to detect laser beam illumination coming from a second transmitter.Further, spike filtering of the background readings may advantageouslybe used to extraneous light noise from the system. It will also beappreciated that the reciprocal of the ratio of the ratio of the sum ofthe reference signals to the difference between the largest and smallestof the reference signals may be computed and the categorization madebased on this reciprocal ratio.

This method is illustrated in greater detail by the flow chart of FIG.4. When a beam has not been acquired and a series of detections arereceived, the maximum strike is detected at 100. The four referencesignal values are calculated at 102, as well as the sum of the fourreference signal values at 104. Next, the signal strength is detectedand, if less than a threshold, the signal is classified as a weak signalthe input capture task is re-initiated. If, however, a sufficientlystrong signal is detected then the reference signals are checked withsignature rules at 106, including for example a determination as towhether the ratio of the sum of the reference signals to the differencebetween the largest and smallest of the reference signals is less than apredetermined level, such as 2.5. If this is the case, then theillumination is classified as a laser beam strike at 108. If the ratiois greater than 2.5, however, the incident is classified as the receiptof a pulse of light from an omnidirectional source, such as a strobe. Ifdesired, an indication can be given that strobe interference isdetected. When the beam strike is detected, the timing of the beamstrike is checked with a phase lock circuit at 110. If the beam strikeis detected outside the expected time frame, then the beam strike isclassified as a illumination from a second laser source. If the beamstrike is detected as occurring within the expected time window,however, it is determined to be a valid beam strike, which may be usedfor position calculation at 112.

It should be appreciated that the present invention is appropriatelyused with any number of arrays, and can be generalized for use withother types of light detection devices. The invention may be used withany detector having multiple arrays, where each array provides twosignals that provide an indication of the detectors in the array thatare illuminated.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

1. A method for detecting a moving, relatively thin beam of laser light,and distinguishing between illumination of photodetectors by such amoving, relatively thin beam of laser light, and illumination ofphotodetectors by an omnidirectional pulse of light, comprising thesteps of: positioning a plurality of photodetector arrays in a generallyvertical row, each array including a plurality of photodetector elementsarranged in a generally vertical array row, such that the photodetectorelements in said arrays are collectively arranged in a generallyvertically oriented, extended row, each of said photodetector elementsproviding an electrical output when illuminated, for each array,providing a portion of the electrical output of each photodetectorelement in the array as a first reference signal related to the spacingof the photodetector element from a first end of the array row, andproviding a portion of the electrical output of each photodetectorelement in the array as a second reference signal related to the spacingof the photodetector element from the second end of said array row,determining the sum of the first and second reference signals from saidphotodetector arrays, determining the difference between the largest ofthe first and second reference signals from said photodetector arraysand the smallest of the first and second reference signals from saidphotodetector arrays, dividing said sum of the first and secondreference signals from said photodetector arrays by said differencebetween the largest of the first and second reference signals from saidphotodetector arrays and the smallest of the first and second referencesignals from said photodetector arrays, thereby providing a quotient,and interpreting the quotient as indicating illumination of saidphotodetectors by a moving, relatively thin beam of laser light whensaid quotient is less than a predetermined level, and interpreting thequotient as indicating illumination of said photodetectors by anomnidirectional pulse of light when said quotient is greater than saidpredetermined level.
 2. The method according to claim 1, furthercomprising the step of displaying an indication of said position of saidreference light with respect to said row.
 3. The method according toclaim 1, in which the step of positioning a plurality of photodetectorarrays in a generally vertically oriented row comprises the step ofevenly spacing said plurality of photodetector arrays along saidgenerally vertically oriented row, and evenly spacing said plurality ofphotodetector elements within each array.
 4. The method according toclaim 1, in which there are four reference signals, and in which thepredetermined level used for interpretation of the quotient isapproximately 2.5.
 5. A method for distinguishing between illuminationof photodetectors by a moving, relatively thin beam of laser light, andillumination of photodetectors by an omnidirectional pulse of light,comprising the steps of: positioning a plurality of photodetectorelements, said elements being connected into a plurality of arrays andeach photodetector element providing an electrical output whenilluminated, for each array providing a first reference signal, and asecond reference signal, the difference between the reference signalsindicating the position of photodetector elements in the array beingilluminated, and determining whether at least some of said referencesignals are below a reference level and interpreting the situation,where fewer than a predetermined number of said reference signals arebelow said reference level as an indication of illumination ofphotodetectors by an omnidirectional pulse of light.
 6. The method ofclaim 5 for distinguishing between illumination of photodetectors by amoving, relatively thin beam of laser light, and illumination ofphotodetectors by an omnidirectional pulse of light, further comprisingthe step of assessing the distribution of the light energy received bysaid photodetectors by comparing the level of energy received by all ofthe photodetectors with the variation among the photodetectors in thelevels of energy received, whereby wide variation in the level of energyas compared to the level of energy received by all of the photodetectorssuggests illumination by a relatively thin beam of laser light, andnarrow variation in the level of energy as compared to the level ofenergy received by all of the photodetectors suggests illumination by anomnidirectional pulse of light.
 7. The method of claim 6 fordistinguishing between illumination of photodetectors by a moving,relatively thin beam of laser light, and illumination of photodetectorsby an omnidirectional pulse of light in which said omnidirectional pulseof light comprises a strobe light pulse.
 8. A method for distinguishingbetween illumination of photodetectors by a moving, relatively thin beamof laser light, and illumination of photodetectors by an omnidirectionalpulse of light, comprising the steps of: positioning a plurality ofphotodetector elements, said elements being connected into a pluralityof arrays and each photodetector element providing an electrical outputwhen illuminated, for each array providing a first reference signal, anda second reference signal, the difference between the reference signalsindicating the position of photodetector elements in the array beingilluminated, and assessing the distribution of the light energy receivedby said photodetectors by comparing the level of energy received by allof the photodetectors with the variation among the photodetectors in thelevels of energy received, whereby wide variation suggests illuminationby a relatively thin beam of laser light and narrow variation suggestsillumination by an omnidirectional pulse of light.
 9. The methodaccording to claim 8 for distinguishing between illumination ofphotodetectors by a moving, relatively thin beam of laser light, andillumination of photodetectors by an omnidirectional pulse of lightincludes the step of determining whether at least some of said referencesignals are below a reference level and interpreting the situation wherefewer than a predetermined number of said reference signals are belowsaid reference level as an indication of illumination of photodetectorsby an omnidirectional pulse of light.
 10. The method according to claim8 for distinguishing between illumination of photodetectors by a moving,relatively thin beam of laser light, and illumination of photodetectorsby an omnidirectional pulse of light in which said omnidirectional pulseof light comprises a strobe light pulse.
 11. A device for detecting amoving, relatively thin beam of laser light, and distinguishing betweenillumination of photodetectors by such a moving, relatively thin beam oflaser light, and illumination of photodetectors by an omnidirectionalpulse of light, comprising: a plurality of photodetector arrays arrangedin a generally vertical row, each array including a plurality ofphotodetector elements arranged in a generally vertical array row, suchthat the photodetector elements in said arrays are arranged in agenerally vertically oriented, extended row, each of said photodetectorelements providing an electrical output when illuminated, a plurality ofcircuits, each such circuit associated with a respective one of saidplurality of photodetector arrays, each circuit providing a portion ofthe electrical output of each photodetector element in the associatedarray as a first reference signal related to the spacing of thephotodetector element from a first end of said array row, and providinga portion of the electrical output of each photodetector element in theassociated array as a second reference signal related to the spacing ofthe photodetector element from the second end of said array row, theweighting circuit associated with each array row being connected to theweighting circuits associated with adjacent array rows, circuitry,associated with and responsive to the reference signals from connectedweighting circuits, for assessing the distribution of the light energyreceived by said photodetectors by comparing the level of energyreceived by all of the photodetectors with the variation among thephotodetectors in the levels of energy received, whereby wide variationsuggests illumination by a relatively thin beam of laser light andnarrow variation suggests illumination by an omnidirectional pulse oflight.
 12. A device according to claim 1 I for detecting a moving,relatively thin beam of laser light, and distinguishing betweenillumination of photodetectors by such a moving, relatively thin beam oflaser light, and illumination of photodetectors by an omnidirectionalpulse of light, in which said circuitry assesses the distribution of thelight energy received by said photodetectors by determining whether somephotodetectors received a significantly reduced level of light.
 13. Adevice according to claim 11 for detecting a moving, relatively thinbeam of laser light, and distinguishing between illumination ofphotodetectors by such a moving, relatively thin beam of laser light,and illumination of photodetectors by an omnidirectional pulse of lightin which said photodetector elements comprise PIN diodes.
 14. A deviceaccording to claim 11 for detecting a moving, relatively thin beam oflaser light, and distinguishing between illumination of photodetectorsby such a moving, relatively thin beam of laser light, and illuminationof photodetectors by an omnidirectional pulse of light, in which saidthe first and second reference signals of the array illuminated by athin beam of laser light provide an indication of the photodetector orphotodetectors illuminated by the beam.